In modern cellular radio systems, the radio network has a strict control on the behavior of a user equipment. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from a base station to the user equipment, UE.
At power-on or after a long standby time, the UE is not synchronized in the uplink. The UE may derive from the downlink (control) signals an uplink frequency and power estimate. However, a timing estimate is difficult to make since the round-trip propagation delay between the base station and the UE is unknown. So even if UE uplink timing is synchronized to the downlink, it may arrive too late at the base station receiver because of the propagation delays. Therefore, before commencing traffic, the UE has to carry out a Random Access (RA) procedure to the network. After the RA, base station can estimate the timing misalignment of the UE uplink and send a correction message. During the RA, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a RA procedure.
Usually, a Physical Random Access Channel (PRACH) is provided for the UE to request access to the network. An access burst is used which contains a preamble with a specific sequence with good autocorrelation properties. The PRACH can be orthogonal to the traffic channels. For example, in GSM a special PRACH slot is defined.
Because multiple UEs may request access at the same time, collisions may occur between requesting UEs. Therefore, multiple RA preambles have been defined for Evolved UTRAN (E-UTRAN), also called for LTE, Long Term Evolution. A UE performing RA picks randomly a preamble out of a pool and transmits it. The preamble represents a random UE ID which is used by the base station when granting the UE access to the network. The base station receiver may resolve RA attempts performed with different preambles and send a response message to each UE using the corresponding random UE IDs. In case that multiple UEs simultaneously use the same preamble a collision occurs and most likely the RA attempts are not successful since the base station cannot distinguish between the two users with a different random UE ID. In LTE, E-UTRAN, sixty four preambles are provided in each cell. Preambles assigned to adjacent cells are typically different to insure that a RA in one cell does not trigger any RA events in a neighboring cell. Information that must be broadcasted is therefore the set of preambles that can be used for RA in the current cell.
One or multiple RA preambles are derived from a single Zadoff-Chu sequence—in the following also denoted root sequence—by cyclic shifting: Due to the ideal auto correlation function of Zadoff-Chu sequence, multiple mutually orthogonal sequences may be derived from a single root sequence by cyclic shifting one root sequence multiple times the maximum allowed round trip time plus delay spread in time-domain. Since each cyclic shift amount must be at least as large as the maximum round trip time in the cell plus delay spread the number of preamble that can be derived from a single root sequence is cell size dependent and decreases with cell size. In order to support operation in cells with different sizes LTE defines sixteen basic cyclic shift lengths supporting cell sizes from approximately 1.5 km up to approximately 100 km. The value that is used in the current cell is broadcasted.
Not only the length of the basic cyclic shift should be larger than the maximum round trip time plus delay spread, also the cyclic prefix and the guard period—which account for the timing uncertainty in unsynchronized RA—should be larger than the maximum round trip time plus delay spread. LTE FDD, Frequency Division Duplex, currently defines four different RA preamble formats with three different cyclic prefix/guard period length supporting cell sizes of 15 km, 30 km, and 100 km.
The cell size that is supported with a certain RA configuration is therefore limited by
1) the length of the cyclic prefix/guard period and
2) the length of the basic cyclic shift.
In addition to these limitations of course also received energy is crucial, some of the RA preamble formats are therefore longer to increase the energy received in the base station. Currently only one set of basic cyclic shift lengths/values is defined, independent which cyclic prefix/guard period or RA preamble format is used. For example, a preamble format with 100 μs cyclic prefix/guard period supports cell sizes up to 15 km. In this case all basic cyclic shift lengths that support larger cell sizes cannot be efficiently used since a supported cell size is limited by the cyclic prefix and/or the size of the guard time and a basic cyclic shift that is longer than the cyclic prefix is an unnecessary over dimensioning.